The proposed study will examine the differences in inhibition in bipolar disorder and ADHD using Gray's (1981, 1987) theory of behavioral activation and inhibition (BAS and BIS) and psychophysiology. This line of research is important because it examines neurological substrates that may be useful in differentiating the neural pathways of the two disorders. The ultimate goal of this research is to build a model that may lead to the differential diagnosis of bipolar disorder and ADHD to be tested subsequently in pediatric samples. Three groups of undergraduates (bipolar, ADHD, and normal control) at Temple University will be administered a diagnostic interview for inclusion in the study. Final sample participants will be administered self-report questionnaires on BIS/BAS, impulsivity, and current mood state. A session using an electroencephalogram (EEG) will involve two computerized tasks: one measuring deficits in inhibition during a continuous performance task, the other measuring deficits in inhibition during a mixed-contingencies task involving conditions of punishment and reward. Analysis of variance techniques and follow-up tests will assess for group differences in BIS/BAS, impulsivity and inhibition, and frontal lobe asymmetries.